I know I'm pretty late to the game with WOWTCG being discontinued, but I finally input all 4800+ of my cards into the collection manager thing. I want to build some decks for 1v1s and raids with friends, but not sure where to start. I used to just build decks to beat my buddy that I played with at lunch, but without a goal in mind, not sure how to build em.

You can just build the decks you want in the TCGBrowser and if you are missing some cards, just use the print icon and you can print out any cards you are missing.

Just remake the deck but only with the cards you DON'T have, that way when you print there are no duplicates.
For sleeving just take some bad card and put it in the sleeve, and slide the printed one in front. The game is dead so there's no issue using the printed proxies, just have some fun.